sicp-2.60.lisp

; No change, as same as before
(define (element-of-set? x set)
  (cond ((null? set) false)
        ((equal? x (car set)) true)
        (else (element-of-set? x (cdr set)))))

; Just use cons, don't need to use element-of-set anymore
(define (adjoin-set x set)
  (cons x set))

; as same as before
(define (intersection-set set1 set2)
  (cond ((or (null? set1) (null? set2)) '())
        ((element-of-set? (car set1) set2)
         (cons (car set1)
               (intersection-set (cdr set1) set2)))
        (else (intersection-set (cdr set1) set2))))

; changed, to use append is enough!
(define (union-set set1 set2) 
  (append set1 set2))